1. Field of the Invention
The present invention relates to a projection exposure apparatus which is used in a lithography process for manufacturing semiconductor elements, imaging elements (CCD), liquid crystal display elements, thin film magnetic heads or the like, for example and in which a mask pattern is transferred onto a photosensitive substrate through a projection optical system.
2. Related Background Art
Projection exposure apparatuses for manufacturing a semiconductor element such as IC, LSI and the like are generally categorized into a projection exposure apparatus of step-and-repeat type in which a reticle as a mask and shot areas of a wafer (or glass plate and the like) as a photosensitive substrate are positioned relative to each other in a predetermined positional relation by using a projection optical system and pattern images on the reticle are collectively transferred to the entirety of each shot area, and a projection exposure apparatus of step-and-scan type in which the pattern images on the reticle are successively transferred to each of the shot areas of the wafer by scan-moving the reticle and the shot areas relative to the projection optical system. These two types are in common in the point that the reticle pattern is projected through the projection optical system. In this regard, it is important how the reticle pattern is correctly projected onto the wafer.
In general, although the projection optical system is designed so that various optical aberrations become substantially "zero" under a predetermined condition, if an atmospheric pressure and/or a temperature around the projection optical system is changed due to the change in the environment during the projection exposure or if heat absorption occurs due to the illumination of exposure illumination light, there will occur the change in index of refraction of fluid existing between lens elements of the projection optical system, expansion of the lens elements, the change in index of refraction of the lens elements, and expansion of lens barrel. As a result, when the reticle pattern is projected onto the wafer, a distortion phenomenon (that the projected image is deviated in a direction perpendicular to an optical axis of the projection optical system) occurs. Such distortion is categorized into a linear error (component that a focusing position is changed with respect to an image height in a linear function manner) and a non-linear error (components other than the linear error), and the linear error is also called as "linear magnification error" (component that the magnification is changed with respect to the image height in a linear function manner). In the past, in order to correct the linear magnification error, there has been utilized a lens control system in which some of lens elements in the projection optical system is driven or pressure of the fluid between some lens elements is controlled.
However, if the non-linear error exists in the projection optical system, in both projection optical systems of step-and-repeat type and of step-and-scan type, the projected image will be distorted, thereby worsening the aligning accuracy.
The present invention aims to eliminate the above-mentioned conventional drawback, and an object of the present invention is to provide a projection exposure apparatus in which the distortion that is worsened by the change in an environmental condition such as atmospheric pressure and a temperature around a projection optical system and the absorption of exposure illumination light (particularly, a non-linear magnification error (higher order error)) can be corrected.
If the above-mentioned change in index of refraction of the fluid existing between the lens elements of the projection optical system, expansion of the lens elements, change in index of refraction of the lens elements and expansion of lens barrel occur, when the reticle pattern is projected onto the wafer, a position (focus position) of an image plane (best focus plane) for the projected image is deviated or displaced in an optical axis of the projection optical system, thereby causing a defocus phenomenon in which the surface of the wafer is deviated out of the image plane. Such defocus is categorized into linear defocus (component that a defocus amount is changed with respect to the image height in a linear function manner) and non-linear defocus (components other than the linear defocus). In the past, in order to correct the linear defocus, there have been proposed an auto focus mechanism in which the focus position of the wafer is controlled along the direction of the image plane and an auto levelling mechanism in which an inclination angle of the wafer is controlled to be aligned with the image plane.
However, if the non-linear defocus such as curvature of field occurs in the image plane of the projection optical system, in both projection optical systems of step-and-repeat type and of step-and-scan type, focal depth of the eventually obtained image plane will be entirely narrowed, thereby deteriorating the resolving power.
The present invention aims to eliminate the above-mentioned conventional drawback, and another object of the present invention is to provide a projection exposure apparatus in which defocus of an image plane of a projection optical system that is worsened by the change in an environmental condition such as atmospheric pressure around the projection optical system, the absorption of exposure illumination light or flexure of a reticle (particularly, non-linear defocus such as curvature of field) can be corrected.
To solve the above problems, it is considered that a temperature of at least one lens element of the projection optical system is adjusted. When the lens element is heated or cooled to achieve a target temperature of the lens element, a predetermined time period is elapsed until the target temperature of the lens element is obtained. Accordingly, if the exposure is started before such time period is elapsed, the exposure will be effected before the imaging characteristic of the lens element is corrected. A further object of the present invention is to provide a projection exposure apparatus in which the exposure is effected after predetermined temperature adjustment is completed.
Regarding the temperature adjustment, not only it takes a long time to achieve the target temperature of the lens element, but also a device for effecting the temperature adjustment is complicated. Accordingly, it is desirable to provide a projection exposure apparatus in which the imaging characteristic can be corrected without the temperature adjustment and the corrected condition is not substantially changed even if environmental conditions are changed. A still further object of the present invention is to provide such a projection exposure apparatus.
During the exposure, when the illumination light having high illumination energy is directed, the imaging characteristic is often gradually changed from an initial condition due to the absorption of the illumination light of the projection optical system. To eliminate this drawback, there have been proposed various methods for keeping the entire temperature of the projection optical system constant in order to maintain the initial condition of the imaging characteristic of the projection optical system. For example, a method in which temperature-adjusted air is supplied around the projection optical system to keep the temperature of the projection optical system constant has been proposed.
In the above-mentioned conventional techniques, since the entire projection optical system is uniformly cooled, the temperature of the projection optical system as a whole can be kept substantially constant. However, due to the difference in light paths for the illumination light beam in the projection optical system caused by the difference in illumination condition and/or the difference in kind of patterns to be exposed, it is inevitable that temperature distribution occurs in the projection optical system. And, as the reduction of a line width achieved by remarkable progress of the recent high density integrated circuits is advanced, the fluctuation in the focusing ability of the projection optical system becomes innegligible more and more.
Further, as the reduction of the line width is advanced, illumination light having a short wavelength (for example, ultraviolet light which can provide high resolving power, far-ultraviolet light such as ArF excimer laser light (having a wavelength of 193 nm) and the like) has been used. However, since such short wavelength includes an oxygen absorption band, there arises a problem that a part of illumination energy of the illumination light which should be used to effect the exposure is absorbed to the oxygen in air. Further, illumination light beams having a wavelength shorter than the ultraviolet zone tend to cause a photochemical reaction for changing the oxygen in air to ozone. The illumination energy of the illumination light is also absorbed to the ozone generated by such a photochemical reaction.
When the illumination light having the wavelength shorter than the ultraviolet zone is used, for example, in an apparatus having a pellicle (dust-proof film) for protecting the pattern area of the reticle, the oxygen is changed to the ozone by the illumination light within a closed space defined by a pellicle frame and the reticle, and the generated ozone tends to be accumulated without dispersion. Thus, there arises a problem that the energy loss due to the absorption of the illumination light is gradually increased.
The present invention aims to eliminate the above-mentioned conventional drawback, and an object of the present invention is to provide a projection exposure apparatus in which illumination energy of exposure illumination light is hard to be absorbed.